Magnetic recording medium, magnetic signal reproduction device and method of manufacturing magnetic recording medium

ABSTRACT

The magnetic recording medium has a magnetic layer containing multiple nonmagnetic particles having a ratio, major axis length/minor axis length, of less than or equal to 1.5, the multiple nonmagnetic particles are present in the magnetic layer in a state where, when the depth to which each of the multiple nonmagnetic particles is embedded in the magnetic layer in observation of a sectional image picked up by SEM is denoted as b and the thickness of the magnetic layer as t, the average value of the ratio of b/t is less than or equal to 0.9, and the number of protrusions 5 nm or greater in height is 800 or greater and the number of protrusions 20 nm or greater in height is 20 or less as measured by AFM per an area 40 μm×40 μm on the magnetic layer side surface of the magnetic recording medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2015-195161 filed on Sep. 30, 2015. The aboveapplication is hereby expressly incorporated by reference, in itsentirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a magnetic recording medium, a magneticsignal reproduction device and a method of manufacturing a magneticrecording medium.

Discussion of the Background

In a particulate magnetic recording medium, a magnetic layer containingferromagnetic powder and binder is present on a nonmagnetic support anda nonmagnetic layer containing nonmagnetic powder and binder isoptionally present between the nonmagnetic support and the magneticlayer.

Conventionally, nonmagnetic powder (nonmagnetic particles) that cancontribute to reducing the coefficient of friction during running hasbeen incorporated along with ferromagnetic powder into the magneticlayer of a particulate magnetic recording medium (for example, seeJapanese Unexamined Patent Publication (KOKAI) No. 2011-48878 or Englishlanguage family members US2011/052908A1 and U.S. Pat. No. 8,535,817,which are expressly incorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

When the coefficient of friction is high in the course of reproducing asignal recorded on a magnetic recording medium while bringing a magnetichead and the surface (on the magnetic layer side) of a magneticrecording medium into (sliding) contact, the increase in noise due toreduced running stability and adhesion to the magnetic head of foreignmaterial generated by shaving of the surface of the magnetic recordingmedium cause output fluctuation (spacing loss). To inhibit theoccurrence of such phenomena, it is desirable to reduce the coefficientof friction. Incorporating the above nonmagnetic particles into themagnetic layer is an effective way of doing this. An increase in thecoefficient of friction during running can be inhibited by reducing thecontact area (the so-called actual contact area) in the course ofbringing the surface of a magnetic recording medium into contact with amagnetic head. These nonmagnetic particles can impart a suitable degreeof roughness to the surface of the magnetic recording medium, and arethus thought to contribute to reducing the actual contact area.

Conversely, when the surface of the magnetic recording medium becomescoarse, a drop in output is produced due to an increase in spacingbetween the magnetic head and the surface of the magnetic recordingmedium during signal reproduction, and the electromagneticcharacteristics (signal-to-noise ratio: SNR) end up dropping. That is,there is a tradeoff between the reduction in the coefficient of frictionduring running and enhancement of the electromagnetic characteristics.It is not easy to achieve both.

In this regard, Japanese Unexamined Patent Publication (KOKAI) No.2011-48878 proposes a means of achieving both a reduction in thecoefficient of friction (described as an “improvement in the frictioncharacteristic” in Japanese Unexamined Patent Publication (KOKAI) No.2011-48878) and good electromagnetic characteristics. However,investigation by the present inventor has shown that it is desirable tofurther improve the magnetic recording medium described in JapaneseUnexamined Patent Publication (KOKAI) No. 2011-48878 to further enhancethe electromagnetic characteristics while achieving a reduction in thecoefficient of friction during running.

An aspect of the present invention provides for a magnetic recordingmedium that is capable of reducing the coefficient of friction duringrunning while further enhancing electromagnetic characteristics.

An aspect of the present invention relates to:

a magnetic recording medium having a magnetic layer containingferromagnetic powder and binder on a nonmagnetic support;

wherein the magnetic layer contains multiple nonmagnetic particleshaving a ratio of the major axis length to the minor axis length (majoraxis length/minor axis length) of less than or equal to 1.5;

the multiple nonmagnetic particles are present in the magnetic layer ina state where, when the depth to which each of the multiple nonmagneticparticles is embedded in the magnetic layer in observation of asectional image picked up by a scanning electron microscope is denotedas b and the thickness of the magnetic layer is denoted as t, theaverage value of the ratio of b/t is less than or equal to 0.9; and

the number of protrusions greater than or equal to 5 nm in height isgreater than or equal to 800 and the number of protrusions greater thanor equal to 20 nm in height is less than or equal to 20 as measured byan atomic force microscope per an area 40 μm×40 μm on the magnetic layerside surface of the magnetic recording medium.

The above magnetic recording medium can make it possible to reduce thecoefficient of friction during running, and to obtain betterelectromagnetic characteristics than those achieved in the magneticrecording medium described in Japanese Unexamined Patent Publication(KOKAI) No. 2011-48878. Although not intended to limit the presentinvention in any way, the present inventor presumes the following inthis regard.

(1) The present inventor presumes that the presence of multiplenonmagnetic particles—which have a ratio of major axis length to minoraxis length (major axis length/minor axis length) of less than or equalto 1.5—in the magnetic layer in a state where, when the depth to whicheach of the multiple nonmagnetic particles is embedded in the magneticlayer is denoted as b and the thickness of the magnetic layer is denotedas t, the average value of the ratio of b/t is less than or equal to 0.9when observing a sectional image picked up by a scanning electronmicroscope, can contribute to further enhancing the electromagneticcharacteristics. The details are as set forth below.

The present inventor presumes that one factor impeding furtherimprovement in the electromagnetic characteristics in the magneticrecording medium described in Japanese Unexamined Patent Publication(KOKAI) No. 2011-48878 is that the nonmagnetic particles that areincorporated into the magnetic layer to reduce the coefficient offriction during running end up accounting for the entire region in thedirection of thickness of the magnetic layer. When the thickness of themagnetic layer is denoted as 1 in the magnetic recording mediumdescribed in Japanese Unexamined Patent Publication (KOKAI) No.2011-48878, these nonmagnetic particles have an average particlediameter of greater than or equal to 1.1 (specifically, greater than orequal to 1.1 but less than or equal to 8.0) (see claim 1 of JapaneseUnexamined Patent Publication (KOKAI) No. 2011-48878). Thus, it isthought to be highly possible that they end up occupying the entireregion in the direction of thickness. If the nonmagnetic particles wereto occupy the entire region in the direction of thickness of themagnetic layer, the ratio b/t of the depth of embedding b to thethickness of the magnetic layer would become 1.0.

By contrast, the average value of the ratio b/t of the depth ofembedding b to the thickness of the magnetic layer in the above magneticrecording medium is less than or equal to 0.9. This means that many ofthe nonmagnetic particles do not account for the entire region of themagnetic layer in the direction of thickness, and that a region ispresent in which the vicinity of the interface with the nonmagneticsupport adjacent to the magnetic layer or with the nonmagnetic layer,described in detail further below, is not accounted for by nonmagneticparticles in the magnetic layer. The present inventor presumes that thepresence of nonmagnetic particles in this state can contribute tofurther enhancing electromagnetic characteristics.

The reason for specifying nonmagnetic particles having a ratio of majoraxis length to minor axis length (major axis length/minor axis length)of less than or equal to 1.5 as the above nonmagnetic particles is tospecify nonmagnetic particles that are capable of contributing toreducing the actual contact area by protruding from the surface of themagnetic recording medium and contributing to a suitable coarseness.

(2) The present inventor presumes that the presence of a number ofprotrusions greater than or equal to 5 nm in height of greater than orequal to 800 and a number of protrusions greater than or equal to 20 nmin height of less than or equal to 20 as measured by an atomic forcemicroscope per an area 40 μm×40 μm on the magnetic layer side surface ofthe magnetic recording medium can contribute to reducing the coefficientof friction during running without impeding further improvement in theelectromagnetic characteristics. More particularly, the present inventorpresumes that reducing the number of protrusions greater than or equalto 20 nm in height that are thought to impede further improvement in theelectromagnetic characteristics and ensuring the suitable presence ofprotrusions greater than or equal to 5 nm in height, which are thoughtto contribute to reducing the actual contact area in the course of themagnetic head coming into contact with the surface of the magneticrecording medium during running without impeding further improvement inthe electromagnetic characteristics, contribute to reducing thecoefficient of friction during running.

However, the above is a mere presumption and is not intended to limitthe present invention in any way.

In one embodiment, the ratio φ/t of the average major axis length φ ofthe multiple nonmagnetic particles having a ratio of major axis lengthto minor axis length (major axis length/minor axis length) of less thanor equal to 1.5 that are contained in the magnetic layer to thethickness of the magnetic layer is less than or equal to 1.0.

In one embodiment, the average major axis length φ of the multiplenonmagnetic particles having a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5 that are contained in the magnetic layer falls within a range of 10nm to 100 nm.

In one embodiment, the thickness of the magnetic layer t is less than orequal to 100 nm.

In one embodiment, the multiple nonmagnetic particles having a ratio ofmajor axis length to minor axis length (major axis length/minor axislength) of less than or equal to 1.5 that are contained in the magneticlayer are inorganic oxide particles.

In one embodiment, the inorganic oxide particles are silicon oxideparticles.

In one embodiment, the silicon oxide particles are silica colloidalparticles.

In one embodiment, the number of protrusions greater than or equal to 5nm in height as measured by an atomic force microscope per an area of 40μm×40 μm on the magnetic layer side surface of the magnetic recordingmedium is greater than or equal to 800 but less than or equal to500,000.

In one embodiment, the number of protrusions greater than or equal to 5nm in height as measured by an atomic force microscope per an area of 40μm×40 μm on the magnetic layer side surface of the magnetic recordingmedium is greater than or equal to 4,000.

In one embodiment, the number of protrusions greater than or equal to 20nm in height as measured by an atomic force microscope per an area of 40μm×40 μm on the magnetic layer side surface of the magnetic recordingmedium is less than or equal to 10.

In one embodiment, the magnetic recording medium has a nonmagnetic layercontaining nonmagnetic powder and binder between the nonmagnetic supportand the magnetic layer.

A further aspect of the present invention relates to a magnetic signalreproduction device including the above magnetic recording medium and amagnetic reproduction head.

In one embodiment, the magnetic signal reproduction device reproduceswith the magnetic reproduction head a magnetic signal that has beenrecorded at a linear recording density of greater than or equal to 200kfci on the above magnetic recording medium.

A further aspect of the present invention relates to a method ofmanufacturing the above magnetic recording medium, including formationof a magnetic layer,

wherein the formation of a magnetic layer includes:

coating and drying a magnetic coating film-forming compositioncontaining ferromagnetic powder and binder, but not containingnonmagnetic particles with a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5, on a nonmagnetic support, either directly or over one or more otherlayers to form a magnetic coating film;

coating and drying an overcoat liquid, containing multiple nonmagneticparticles having a ratio of major axis length to minor axis length(major axis length/minor axis length) of less than or equal to 1.5 andsolvent, on the above magnetic coating film to dispose the nonmagneticparticles on the above magnetic coating film; and

pressing into the magnetic coating film the nonmagnetic particles thathave been disposed to form a magnetic layer that contains ferromagneticpowder and binder and in which the above nonmagnetic particles arepresent in the above-described state.

In the present invention, the statement that the magnetic coatingfilm-forming composition “does not contain” nonmagnetic particles with aratio of major axis length to minor axis length (major axis length/minoraxis length) of less than or equal to 1.5 means that they are notactively employed as a component in the course of preparing thecomposition, and that unintentional mixing in is permissible. Forexample, the mixing in of a quantity of about less than or equal to 0.01weigh part per 100 weight parts of ferromagnetic powder contained in theabove magnetic coating film-forming composition is permissible.

An aspect of the present invention can provide a magnetic recordingmedium with a low coefficient of friction during running that canfurther enhance electromagnetic characteristics.

A further aspect of the present invention can provide a magnetic signalreproduction device including the above magnetic recording medium and amethod of manufacturing the above magnetic recording medium.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Unless otherwise stated, a reference to a compound or component includesthe compound or component by itself, as well as in combination withother compounds or components, such as mixtures of compounds.

As used herein, the singular forms “a,” “an,” and “the” include themultiple reference unless the context clearly dictates otherwise.

Except where otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention. Atthe very least, and not to be considered as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding conventions.

Additionally, the recitation of numerical ranges within thisspecification is considered to be a disclosure of all numerical valuesand ranges within that range. For example, if a range is from about 1 toabout 50, it is deemed to include, for example, 1, 7, 34, 46.1, 23.7, orany other value or range within the range.

The following preferred specific embodiments are, therefore, to beconstrued as merely illustrative, and non-limiting to the remainder ofthe disclosure in any way whatsoever. In this regard, no attempt is madeto show structural details of the present invention in more detail thanis necessary for fundamental understanding of the present invention; thedescription making apparent to those skilled in the art how severalforms of the present invention may be embodied in practice.

Magnetic Recording Medium

An aspect of the present invention relates to:

a magnetic recording medium having a magnetic layer containingferromagnetic powder and binder on a nonmagnetic support;

wherein the magnetic layer contains multiple nonmagnetic particleshaving a ratio of the major axis length to the minor axis length (majoraxis length/minor axis length) of less than or equal to 1.5;

the multiple nonmagnetic particles are present in the magnetic layer ina state where, when the depth to which each of the multiple nonmagneticparticles is embedded in the magnetic layer in observation of asectional image picked up by a scanning electron microscope is denotedas b and the thickness of the magnetic layer is denoted as t, theaverage value of the ratio of b/t is less than or equal to 0.9; and

the number of protrusions greater than or equal to 5 nm in height isgreater than or equal to 800 and the number of protrusions greater thanor equal to 20 nm in height is less than or equal to 20 as measured byan atomic force microscope per an area 40 μm×40 μm on the magnetic layerside surface of the magnetic recording medium.

The above magnetic recording medium will be described in detail below.

In the present invention and present specification, the term“nonmagnetic powder” means an aggregation of multiple nonmagneticparticles. The term “aggregation” is not limited to forms in which theconstituent particles are in direct contact, but includes forms in whichbinder, additives, or the like described below is present between theparticles. The term “particles” is also sometimes used to denote powder.These points are also applied to the various powders described in thepresent invention and present specification, such as ferromagneticpowder.

<Average Value of Ratio b/t>

Multiple nonmagnetic particles with a ratio of major axis length tominor axis length (major axis length/minor axis length) of less than orequal to 1.5 are contained in the above magnetic recording medium. Thesemultiple nonmagnetic particles are present in the magnetic layer in astate where, when the depth of embedding of the individual nonmagneticparticles as observed in a sectional image picked up by a scanningelectron microscope (SEM) is denoted as b and the thickness of themagnetic layer is denoted as t, the average value of the ratio b/t isless than or equal to 0.9. The present inventor presumes that having theaverage value of the ratio b/t be less than or equal to 0.9 cancontribute to achieving better electromagnetic characteristics thanthose achieved in Japanese Unexamined Patent Publication (KOKAI) No.2011-48878. The presumptions of the present inventor in this regard areas set forth above. From the perspective of achieving even betterelectromagnetic characteristics, the average value of the ratio b/t isdesirably less than or equal to 0.8, preferably less than or equal to0.7, more preferably less than or equal to 0.6, and still morepreferably, less than or equal to 0.5. The average value of the ratiob/t can be, for example, greater than or equal to 0.2 or greater than orequal to 0.3 However, from the perspective of enhancing theelectromagnetic characteristics, a low average value of ratio b/t isdesirable. It can thus fall below these levels.

(Measurement Methods)

The average value of the ratio b/t set forth above is determined by thefollowing method. An example of measurement in a magnetic recordingmedium in which a nonmagnetic layer is present on a nonmagnetic supportand a magnetic layer is present on the nonmagnetic layer will be givenbelow. However, a magnetic recording medium in which the magnetic layeris directly present on the nonmagnetic support is also possible. In thatcase, the interface that is described below is the interface of themagnetic layer and nonmagnetic support.

(1) Preparing a Sample for Sectional Observation

A sample for sectional observation is prepared by cutting the sample outat a randomly determined position on the magnetic recording medium forwhich the average value of ratio b/t is being determined. The sample forsectional observation is prepared by processing with a focused ion beam(FIB) using a gallium ion (Ga⁺) beam. A specific example of thepreparation method will be given in Examples further below.

(2) Specifying the Observation Region

The sample for sectional observation that has been prepared is observedby SEM and a sectional image (SEM image) is picked up. A field emissionscanning electron microscope (FE-SEM) is employed as the scanningelectron microscope. For example, an FE-SEM S4800 made by Hitachi Ltd.can be employed. An FE-SEM was employed in Examples further below.

In a single sample for sectional observation, a total of four images areobtained by imaging by SEM positions that have been randomly selected inaddition to selected points:

(i) such that there is no overlap in the scope of the images picked up;(ii) such that the magnetic layer side surface (magnetic layer surface)and the nonmagnetic particles protruding on the magnetic layer sidesurface are included in the SEM image; and(iii) such that the entire region in the direction of thickness of themagnetic layer of the sample for sectional observation (that is, theregion from the magnetic layer side surface through the interface of themagnetic layer and the nonmagnetic layer) and the nonmagnetic particlesare included in the SEM image when nonmagnetic particles are presentthat run through the interface of the magnetic layer and the nonmagneticlayer.

The above SEM images are secondary electron (SE) images that are pickedup at an acceleration voltage of 5 kV at an imaging magnification of100,000-fold with 960 vertical pixels by 1,280 horizontal pixels.

In the SEM images, ferromagnetic powder and nonmagnetic powder(nonmagnetic particles) can be readily distinguished by differences inshading on the image, differences in size, differences in shape, and thelike. The materials constituting nonmagnetic particles can be analyzedby energy dispersive X-ray spectrometry (EDS), Auger electronspectroscopy (AES), mapping of components on the surface of the magneticlayer by a scanning electron microscope, and other known methodspermitting compositional analysis.

(3) Measurement of Thickness of Magnetic Layer t

The SEM image that has been picked up is entered into the image analysissoftware WinRoof produced by Mitani Corporation and portions(observation regions) of the magnetic layer are selected within the SEMimage. In selecting the regions to be observed, the length in the widthdirection of the observation region is made the full width of the SEMimage that has been picked up. The term “width direction” that has beenmentioned in relation to the SEM image refers to the width direction ofthe sample for sectional observation that has been imaged. The term“width direction” in the sample for sectional observation is thedirection of width of the magnetic recording medium from which thesamples has been cut. The same applies to the thickness direction.

As regards the thickness direction, the interface between the magneticlayer and the nonmagnetic layer is specified by the following method.Image brightness data in the thickness direction (comprised of threecomponents in the form of a coordinate in the thickness direction, acoordinate in the width direction, and the brightness) are prepared bydigitizing the SEM image. In digitization, the SEM image is divided into1,280 parts in the width direction and the brightness is processed with8 bits to obtain 256-gradation data. The image brightness of each of thecoordinate points that have been separated out is converted to aprescribed gradation value. Next, in the image brightness data that havebeen obtained, the average value of the brightness in the widthdirection at the various coordinate points in the thickness direction(that is, the average value of the brightness at the 1,280 individualcoordinate points that have been divided out) is plotted on the Y-axis,and a brightness curve is plotted with the coordinate in the thicknessdirection being plotted on the X-axis. The differential of thebrightness curve that has been plotted is taken to plot a differentialcurve. The coordinate of the boundary of the magnetic layer and thenonmagnetic layer is specified from the peak position of thedifferential curve that has been plotted. The position corresponding tothe specified coordinate on the SEM image is adopted as the interfacebetween the magnetic layer and the nonmagnetic layer.

The magnetic layer side surface is also specified with the brightnesscurve in the same manner as described above.

At one arbitrary point on each of the four images that have been pickedup, the shortest distance of a straight line between the specifiedmagnetic layer side surface and the interface between the magnetic layerand the nonmagnetic layer is determined, and the average value of thevalues determined for four images is adopted as the thickness of themagnetic layer t. In the present invention and present specification,the term “average value” refers to the arithmetic average.

(4) Specifying Nonmagnetic Particles with a Ratio of Major Axis Lengthto Minor Axis Length (Major Axis Length/Minor Axis Length) of Less thanor Equal to 1.5

A portion of the nonmagnetic particles in each of the four images pickedup is specified based on the image brightness data, and the major axislength and minor axis length are determined by image analysis.Specifically, each (SEM) image is digitized and image brightness dataare produced in the major axis direction and minor axis direction of thenonmagnetic particles (comprised of the three components of a coordinatein the major axis direction, a coordinate in the minor axis direction,and brightness). In the digitization, the SEM image is divided into1,280 parts in the width direction, the brightness is processed with 8bits to obtain 256-gradation data, and the image brightness of eachcoordinate point that has been divided out is converted to a prescribedgradation value. Next, in the image brightness data obtained, thecoordinate in the direction corresponding to the major axis of theparticle is plotted on the X-axis and the average value of thebrightness at each coordinate point in the major axis direction (thatis, the average value of the brightness at each coordinate point dividedinto 1,280) is plotted on the Y-axis to plot a brightness curve. Thedifferential of the brightness curve that has been plotted is taken toplot a differential curve. The coordinate of the boundary of thenonmagnetic particle portion is specified from the peak position of thedifferential curve that has been plotted. The operation of plotting thecoordinate in the direction corresponding to the major axis of theparticle on the X-axis is repeated three times, and the longest axislength is adopted as the major axis length. In the minor axis directionas well, the operation of plotting the coordinate on the X-axis isrepeated three times, and the shortest axis length is adopted as theminor axis length. More particularly, the axis (straight line) on whichthe greatest length can be plotted for the length of the particle isdetermined to be the major axis, and the length of the major axis isadopted as the major axis length. Additionally, the axis of the greatestlength when the particle length is plotted on a straight lineperpendicular to the major axis is determined to be the minor axis, andthe length of this axis is adopted as the minor axis length. However,nonmagnetic particles of indeterminate shape for which a major axisand/or a minor axis constituting the particle cannot be specified fromthe shape are excluded from measurement. For nonmagnetic particles forwhich the ratio of the major axis length and the minor axis length(major axis length/minor axis length) clearly exceeds 1.5 at firstglance, it is acceptable not to measure the ratio of the major axislength and the minor axis length (major axis length/minor axis length).At the edge of the image, nonmagnetic particles that are not completelycontained within the image will sometimes be observed. Such nonmagneticparticles are excluded from measurement.

As set forth above, all of the nonmagnetic particles having a ratio ofthe major axis length to the minor axis length (major axis length/minoraxis length) of less than or equal to 1.5 are specified in each image.

(5) Calculating the Average Value of Ratio b/t

The position where the distance on a straight line from the interface ofthe magnetic layer and the nonmagnetic layer is the shortest isspecified on the contour of each nonmagnetic particle having a ratio ofmajor axis length to minor axis length (major axis length/minor axislength) of less than or equal to 1.5 as specified in (4) above. Thestraight line distance between this position and the interface of themagnetic layer and the nonmagnetic layer is adopted as “c”. In an imagecontaining these nonmagnetic particles, “c” is subtracted from the valueof the thickness t of the magnetic layer obtained in (3), and this valueis adopted as the depth of embedding b (that is, “t−c”=b). For anonmagnetic particle that is fully embedded in the magnetic layerwithout protruding from the magnetic layer side surface, if the ratio ofthe major axis length and minor axis length (major axis length/minoraxis length) is less than or equal to 1.5, the depth of embedding iscalculated by the above method. The value of b that has been calculatedis then divided by the thickness t of the magnetic layer that has beencalculated in (3) for the image containing the nonmagnetic particle tocalculate b/t.

The calculation of b/t is performed for all nonmagnetic particles havinga ratio of major axis length to minor axis length (major axislength/minor axis length) of less than or equal to 1.5 that have beenspecified for the four images that have been picked up. The arithmeticaverage of the values obtained is adopted as the average value of ratiob/t.

Here, b, c, and t are all values expressed in the same unit. Forexample, if μm is adopted as the unit, b, c, and t will all be numericalvalues expressed in μm units. If nm is adopted as the unit, b, c, and twill all be numerical values expressed in nm units. A single unit isalso adopted when calculating the ratio of major axis length to minoraxis length (major axis length/minor axis length).

<Number of Protrusions>

In the above magnetic recording medium, multiple nonmagnetic particleshaving a ratio of major axis length to minor axis length (major axislength/minor axis length) of less than or equal to 1.5 in the magneticlayer are present in the magnetic layer in a state where the averagevalue of b/t is less than or equal to 0.9. Further, the number ofprotrusions measured by atomic force microscope (AFM) on the magneticlayer side surface per an areas of 40 μm×40 μm that are greater than orequal to 5 nm in height is greater than or equal to 800, and the numberof protrusions greater than or equal to 20 nm in height is less than orequal to 20. Thus, it becomes possible to reduce the coefficient offriction drops during running and further improve electromagneticcharacteristics. In this regard, the presumptions of the presentinventor are as set forth above. Unless specifically stated otherwise,the number of protrusions set forth below refers to the number per 40μm×40 μm area.

The number of protrusions greater than or equal to 5 nm in height per 40μm×40 μm area is greater than or equal to 800. From the perspective ofmore effectively inhibiting a rise in the coefficient of friction duringrunning, this number is desirably greater than or equal to 900,preferably greater than or equal to 1,000, more preferably greater thanor equal to 2,000, still more preferably greater than or equal to 3,000,yet more preferably greater than or equal to 4,000, and even morepreferably, greater than or equal to 5,000. The larger the number ofprotrusions greater than or equal to 5 nm in height, the more the risein the coefficient of friction during running can be inhibited.Additionally, the number of protrusions greater than or equal to 5 nm inheight per 40 μm×40 μm area can be less than or equal to 500,000, forexample. It is also possible to exceed this number.

The number of protrusions greater than or equal to 20 nm in height per40 μm×40 μm area is less than or equal to 20. From the perspective offurther enhancing the electromagnetic characteristics, this number isdesirably less than or equal to 15, preferably less than or equal to 10,more preferably less than or equal to 5, still more preferably less thanor equal to 3, and even more preferably, 0.

(The Measurement Method)

The numbers of protrusions set forth above are determined by thefollowing method.

A surface where convex components are equal in volume to concavecomponents in the field of view as measured by AFM is determined as areference surface. Portions greater than or equal to 5 nm in height fromthe reference surface are deemed to be protrusions of greater than orequal to 5 nm in height and portions greater than or equal to 20 nm inheight from the reference surface are deemed to be protrusions greaterthan or equal to 20 nm in height in measurement regions (area of eachmeasurement region: 40 μm×40 μm) in five spots randomly selected on themagnetic layer side surface of the magnetic recording medium that is tobe measured. The number of protrusions in each of the measurementregions is determined. Protrusions that are not fully contained withinthe measurement region may be present along the edges of the measurementregions, and such protrusions are also counted. The number ofprotrusions is counted as a cumulative number, so those protrusions thatare greater than or equal to 5 nm in height will include the protrusionsthat are greater than or equal to 20 nm in height. The followingmeasurement conditions are an example of measurement conditions by AFM.The numbers of protrusions given in Examples further below are theaverage values of the measurement values determined in five spots bymeasurement based on the following measurement conditions.

Regions 40 μm×40 μm in area that had been randomly selected in fivespots on the magnetic layer side surface of a magnetic recording mediumwere measured by AFM (Nanoscope 4 made by Veeco Corp.). The scan rate(probe displacement rate) was 40 μm/sec, and the resolution was 512pixels×512 pixels.

<Methods of Adjusting the Average Value of Ratio of b/t and the Numberof Protrusions>

Methods of adjusting the various values set forth above will bedescribed next.

The number of protrusions greater than or equal to 5 nm in height andthe number of protrusions greater than or equal to 20 nm in height canbe desirably adjusted by means of the protrusion state from the magneticlayer side surface of the magnetic recording medium of nonmagneticparticles having a ratio of major axis length to minor axis length(major axis length/minor axis length) of less than or equal to 1.5.

To adjust the average ratio of b/t to less than or equal to 0.9,nonmagnetic particles having a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5 are desirably not embedded within the magnetic layer to the degreethat they reach the region in the vicinity of the interface between themagnetic layer and the adjacent nonmagnetic support or nonmagneticlayer. In this regard, as described in Japanese Unexamined PatentPublication (KOKAI) No. 2011-48878, so that nonmagnetic particles thatare large relative to the thickness of the magnetic layer are notembedded within the magnetic layer, these nonmagnetic particles are madeto protrude substantially from the magnetic layer side surface of themagnetic recording medium. However, the protruding nonmagnetic particlesare thought to become protrusions greater than or equal to 20 nm inheight on the magnetic layer side surface, causing the number ofprotrusions greater than or equal to 20 nm in height to exceed 20.Additionally, when nonmagnetic particles that are small relative to thethickness of the magnetic layer are simply employed as components of themagnetic layer-forming composition, the nonmagnetic particles arethought to end up being completely embedded in the magnetic layer,making it difficult to achieve a number of protrusions greater than orequal to 5 nm in height of greater than or equal to 800.

Based on the above, the present inventor conducted extensive research.As a result, he conceived that by using nonmagnetic particles that werenot as large as the thickness of the magnetic layer described inJapanese Unexamined Patent Publication (KOKAI) No. 2011-48878, and byemploying a manufacturing method that localized the nonmagneticparticles on the magnetic layer side surface of the magnetic recordingmedium to manufacture a magnetic recording medium, it would be possibleto obtain a magnetic recording medium in which the ratio of b/t was lessthan or equal to 0.9, and in which the number of protrusions greaterthan or equal to 5 nm in height and the number of protrusions greaterthan or equal to 20 nm in height fell within the ranges set forth above.From this perspective, the ratio φ/t of the average value (average majoraxis length) φ of the major axis length of nonmagnetic particles inwhich the ratio of the major axis length to the minor axis length (majoraxis length/minor axis length) is less than or equal to 1.5, to thethickness t of the magnetic layer is desirably less than or equal to1.0, preferably less than or equal to 0.8, and more preferably, lessthan or equal to 0.6.

In recent years, the recording wavelength has tended to become evershorter as the recording density has increased. As the recordingwavelength has shortened, the bit length, which is the minimum recordingunit in magnetic recording, has tended to decrease. At the same time,the impact of the nonmagnetic particles that are contained in the bitvolume (bit length×thickness of the magnetic layer) on theelectromagnetic characteristics has increased. For this reason, as well,it is desirable for the nonmagnetic particles not to account for theentire region in the direction of thickness of the magnetic layer.

Further, the ratio of φ/t can be, for example, greater than or equal to0.1 or greater than or equal to 0.2, or can fall below these levels. Theaverage value (average major axis length) φ of the above major axislength is the arithmetic average of the major axis length of all of thenonmagnetic particles with a ratio of the major axis length to the minoraxis length (major axis length/minor axis length) as specified for 4images picked up by SEM by the method set forth above. The thickness tof the magnetic layer is a value determined by the method set forthabove. In calculating φ/t, the same unit is used for both φ and t.

A desirable example of a method for manufacturing the magnetic recordingmedium will be given further below.

The magnetic layer, nonmagnetic layer and the like of the magneticrecording medium according to an aspect of the present invention will bedescribed in greater detail.

<Magnetic Layer>

(Ferromagnetic Powder)

The various powders that are commonly employed as ferromagnetic powdersin the magnetic layer of magnetic recording media can be employed as theferromagnetic powder. The use of a ferromagnetic powder of small averageparticle size is desirable from the perspective of increasing therecording density of the magnetic recording medium. For this reason, itis desirable to employ a ferromagnetic powder with an average particlesize of less than or equal to 50 nm as the ferromagnetic powder. Fromthe perspective of stability of magnetization, the average particle sizeof the ferromagnetic powder is desirably greater than or equal to 10 nm.

Ferromagnetic hexagonal ferrite powder is a specific example ofdesirable ferromagnetic powder. From the perspectives of achievinghigher density recording and magnetization stability, the averageparticle size (for example, average plate diameter) of ferromagnetichexagonal ferrite powder desirably ranges from 10 nm to 50 nm,preferably 20 nm to 50 nm. Reference can be made to Japanese UnexaminedPatent Publication (KOKAI) No. 2011-225417, paragraphs 0012 to 0030,Japanese Unexamined Patent Publication (KOKAI) No. 2011-216149,paragraphs 0134 to 0136, and Japanese Unexamined Patent Publication(KOKAI) No. 2012-204726, paragraphs 0013 to 0030, for details onferromagnetic hexagonal ferrite powder. The contents of the abovepublications are expressly incorporated herein by reference in theirentirety.

Ferromagnetic metal powder is also a specific example of desirableferromagnetic powder. From the perspectives of achieving higher densityrecording and magnetization stability, the average particle size (forexample, average major axis length) of ferromagnetic metal powderdesirably ranges from 10 nm to 50 nm, preferably 20 nm to 50 nm.Reference can be made to Japanese Unexamined Patent Publication (KOKAI)No. 2011-216149, paragraphs 0137 to 0141, and Japanese Unexamined PatentPublication (KOKAI) No. 2005-251351, paragraphs 0009 to 0023, fordetails on ferromagnetic metal powder. The contents of the abovepublications are expressly incorporated herein by reference in theirentirety.

The average particle size of various powders such as ferromagneticpowder is a value measured with a transmission electron microscope bythe following method unless specifically stated otherwise.

Powder is photographed at a magnification of 100,000-fold with atransmission electron microscope, and the photograph is printed on printpaper at a total magnification of 500,000-fold to obtain a photograph ofthe particles constituting the powder. A target particle is selectedfrom the photograph of particles that has been obtained, the contour ofthe particle is traced with a digitizer, and the size of the (primary)particle is measured. The term “primary particle” refers to anunaggregated, independent particle. The particle size of nonmagneticparticles that are employed for forming a magnetic layer is a valuemeasured by the above method.

The above measurement is conducted on 500 randomly extracted particles.The arithmetic average of the particle size of the 500 particlesobtained in this manner is adopted as the average particle size of thepowder. A Model H-9000 transmission electron microscope made by Hitachican be employed as the above transmission electron microscope, forexample. The particle size can be measured with known image analysissoftware, such as KS-400 image analysis software from Carl Zeiss.

In the present invention and present specification, the average particlesize of the powder, such as ferromagnetic powder and various kinds ofpowder, is the average particle size as obtained by the above methodunless specifically stated otherwise. The average particle sizeindicated in Examples further below was obtained using a Model H-9000transmission electron microscope made by Hitachi and KS-400 imageanalysis software made by Carl Zeiss. The average major axis length φ ofnonmagnetic particles having a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5 that are contained in the magnetic layer is a value that isdetermined in a SEM image by the method set forth above.

The method described in paragraph 0015 of Japanese Unexamined PatentPublication (KOKAI) No. 2011-048878, which is expressly incorporatedherein by reference in its entirety, for example, can be employed as themethod of collecting sample powder from a magnetic recording medium forparticle size measurement.

In the present invention and present specification, unless specificallystated otherwise, the size of the particles constituting powder(particle size) is denoted as follows based on the shape of theparticles observed in the above particle photograph:

(1) When acicular, spindle-shaped, or columnar (with the height beinggreater than the maximum diameter of the bottom surface) in shape, theparticle size is denoted as the length of the major axis constitutingthe particle, that is, the major axis length.(2) When platelike or columnar (with the thickness or height beingsmaller than the maximum diameter of the plate surface or bottomsurface) in shape, the particle size is denoted as the maximum diameterof the plate surface or bottom surface.(3) When spherical, polyhedral, of unspecific shape, or the like, andthe major axis constituting the particle cannot be specified from theshape, the particle size is denoted as the diameter of an equivalentcircle. The term “diameter of an equivalent circle” means that obtainedby the circle projection method.

The “average acicular ratio” of a powder refers to the arithmeticaverage of values obtained for the above 500 particles by measuring thelength of the minor axis, that is the minor axis length, of theparticles measured above, and calculating the value of the (major axislength/minor axis length) of each particle. Unless specifically statedotherwise, the term “minor axis length” refers to, in the case of theparticle size definition of (1), the length of the minor axisconstituting the particle; in the case of (2), the thickness or height,and in the case of (3), since the major axis and minor axis cannot bedistinguished, (major axis length/minor axis length) is deemed to be 1for the sake of convenience.

Unless specifically stated otherwise, when the particle has a specificshape, such as in the particle size definition of (1) above, the averageparticle size is the average major axis length. In the case of (2), theaverage particle size is the average plate diameter, with the averageplate ratio being the arithmetic average of (maximum diameter/thicknessor height). For the definition of (3), the average particle size is theaverage diameter (also called the average particle diameter).

The content (fill rate) of ferromagnetic powder in the magnetic layerdesirably falls within a range of 50 to 90 weight %, and preferablyfalls within a range of 60 to 90 weight %. A high fill rate is desirablefrom the perspective of increasing the recording density.

(Binder)

The above magnetic recording medium contains binder along withferromagnetic powder in the magnetic layer. Polyurethane resins,polyester resins, polyamide resins, vinyl chloride resins, acrylicresins such as those provided by copolymerizing styrene, acrylonitrile,methyl methacrylate and the like, cellulose resins such asnitrocellulose, epoxy resins, phenoxy resins, polyvinylacetal,polyvinylbutyral, and other polyvinyl alkylal resins can be employedsingly, or as mixtures of multiple resins, as the binder contained inthe magnetic layer. Among these, desirable resins are polyurethaneresin, acrylic resins, cellulose resins, and vinyl chloride resins.These resins can be homopolymer or copolymer. These resins can also beemployed as binders in the nonmagnetic layer and in the backcoat layerdescribed further below. Reference can be made to paragraphs 0028 to0031 of Japanese Unexamined Patent Publication (KOKAI) No. 2010-24113,which is expressly incorporated herein by reference in its entirety,with regard to the above binders. At least one curing agent can also beemployed together with the resin suitable for use as the binder. Thecuring agent is a compound having at least one, and desirably two ormore, crosslinking functional groups per molecule. A curing agent havingtwo or more isocyanate groups per molecule (polyisocyanate) is suitable.Reference can be made to Japanese Unexamined Patent Publication (KOKAI)No. 2011-216149, paragraphs 0124 and 0125, for details regardingpolyisocyanate. The curing agent can be employed, for example, by addinga quantity of 0 to 80.0 weight parts, and from the perspective ofincreasing the strength of various layers such as a magnetic layer,desirably 50.0 to 80.0 weight parts, per 100.0 weight parts of thebinder.

(Nonmagnetic Particles Having a Ratio of Major Axis Length to Minor AxisLength (Major Axis Length/Minor Axis Length) of Less than or Equal to1.5)

The above magnetic recording medium contains multiple nonmagneticparticles having a ratio of major axis length to minor axis length(major axis length/minor axis length) of less than or equal to 1.5 inthe magnetic layer in addition to ferromagnetic powder and binder. Thesenonmagnetic particles are present in the magnetic layer in a state wherethe average value of the ratio of b/t is less than or equal to 0.9 andthe number of protrusions greater than or equal to 5 nm in height andthe number of protrusions greater than or equal to 20 nm in height onthe surface on the magnetic layer side fall within the respective rangesset forth above. Thus, the coefficient of friction during running can bereduced and it becomes possible to achieve good electromagneticcharacteristics. The ratio of the major axis length to the minor axislength (major axis length/minor axis length) of the nonmagneticparticles is desirably less than or equal to 1.3, preferably less thanor equal to 1.1, and can be 1.0. When the ratio of the major axis lengthto the minor axis length (major axis length/minor axis length) of thenonmagnetic particles is 1.0, the major axis length and minor axislength of the particles are equal, that is, the particle is a sphere.

As set forth above, the average major axis length φ of the nonmagneticparticles relative to the thickness t of the magnetic layer is desirablysuch that the ratio of φ/t is less than or equal to 1.0. The averagemajor axis length φ desirably falls within a range of 10 nm to 100 nm.However, it suffices for the nonmagnetic particles to be present in themagnetic layer in a state where the average value of the ratio of b/t isless than or equal to 0.9. The average major axis length φ is notlimited to within the range set forth above.

From the perspective of further enhancing electromagneticcharacteristics, the variation in the particle size (major axis length)of the nonmagnetic particles is desirable low. The degree of variationin the major axis length can be evaluated based on the coefficient ofvariation (CV value). The CV value (unit: %)=(σ/φ)×100. As above, φdenotes the average major axis length and σ denotes the standarddeviation in the major axis length. The CV value of nonmagneticparticles with a ratio of major axis length to minor axis length (majoraxis length/minor axis length) of less than or equal to 1.5 is, forexample, less than 30.0%, desirably equal to or less than 15.0%, andpreferably less than or equal to 7.0%. The CV value can be, for example,greater than or equal to 3.0%. Because it is desirable to reduce thevariation in particle size of the nonmagnetic particles, it can also belower than the above value. Colloidal particles are desirable asnonmagnetic particles with a low CV value. In the present invention andpresent specification, the term “colloidal particles” refers toparticles that can be dispersed without settling to prepare a colloidaldispersion when one gram is added per 100 mL of at least one organicsolvent containing at least methyl ethyl ketone, cyclohexanone, toluene,or ethyl acetate, or a mixed solvent of two or more of these solvents inany blending ratio. Provided that the nonmagnetic particles that havebeen employed in the formation of the magnetic layer are available, thefact that the nonmagnetic particles contained in the magnetic layer arecolloidal particles can be evaluated by whether the nonmagneticparticles have the properties that correspond to the above definition ofcolloidal particles. Nonmagnetic particles that have been extracted fromthe magnetic layer can also be used to evaluate whether they have theproperties corresponding to the above definition of colloidal particles.The nonmagnetic particles can be extracted from the magnetic layer bythe following method, for example.

1. Scraping off about 1 g of magnetic layer. The scraping can be donewith a razor blade, for example.2. Placing the magnetic layer sample that has been obtained by scrapingin a container such as an eggplant-shaped flask and adding 100 mL oftetrahydrofuran to the container. Tetrahydrofuran comes in the form ofcommercial products to which a stabilizer has been added, and in theform of commercial products without stabilizer. A tetrahydrofuran towhich no stabilizer has been added is used. The same applies to thetetrahydrofuran that is employed for washing further below.3. Mounting a reflux condenser on the container and heating the contentsfor 90 minutes in a bath at a water temperature of 60° C. Filtering withfilter paper the contents of the container after heating, washingseveral times with tetrahydrofuran the solid component that has remainedon the filter paper, and transferring the solid component to a containersuch as a beaker following washing. 4N (4 mol/L) hydrochloric acidaqueous solution is added to the container and non-dissolved residue iscollected by filtration with a filter. A filter with a pore diametersmaller than 0.05 μm is employed. For example, a membrane filter (suchas an MF Millipore made by Merck Corp.) that is employed inchromatographic analysis can be employed. Washing the residue that hasbeen removed by filtration with a filter several times with pure waterand then drying it.

Dissolving the ferromagnetic powder and organic material (binder and thelike) obtained by the above operation and recovering the nonmagneticparticles as a residue.

Nonmagnetic particles can be extracted from the magnetic layer by theabove process. When multiple types of nonmagnetic particles arecontained in the nonmagnetic particles that are thus extracted, themultiple types of nonmagnetic particles can be separated by differencesin density.

For the nonmagnetic particles (so-called protrusion-forming agent)employed as components of the magnetic layer to form protrusions on themagnetic layer side surface of the magnetic recording medium, theaverage particle size as measured by the method for measuring theaverage particle size of various powders set forth above and thecoefficient of variation CV value that is calculated from the valuesmeasured by this method are desirably within the ranges set forth abovefor various values determined by observation of a sectional image pickedup by SEM.

The nonmagnetic particles with a ratio of major axis length to minoraxis length (major axis length/minor axis length) of less than or equalto 1.5 can be organic or inorganic particles, with inorganic particlesbeing desirable. Examples of inorganic particles are particles such asmetal oxides, metal carbonates, metal sulfates, metal nitrides, metalcarbides, and metal sulfates. Specific examples are α-alumina with an aconversion rate of greater than or equal to 90%, β-alumina, γ-alumina,θ-alumina, silicon oxide such as silicon dioxide, silicon carbide,chromium oxide, cerium oxide, α-iron oxide, goethite, corundum, siliconnitride, titanium carbide, titanium dioxide, tin oxide, magnesium oxide,tungsten oxide, zirconium oxide, born nitride, zinc oxide, calciumcarbonate, calcium sulfate, barium sulfate, and molybdenum disulfide, aswell as composite oxides of two or more of the above.

As inorganic particles, inorganic oxide particles are desirable, andsilicon oxide particles are preferred.

As set forth above, one desirable form of the nonmagnetic particles isthat of colloidal particles. Colloidal particles in the form ofinorganic colloidal particles are desirable from the perspective ofavailability, with inorganic oxide colloidal particles being preferred.Colloidal particles of the above inorganic oxide are examples ofinorganic oxide colloidal particles. Examples of colloidal particlesalso include composite inorganic oxide colloidal particles such asSiO₂.Al₂O₃, SiO₂.B₂O₃, TiO₂.CeO₂, SnO₂.Sb₂O₃, SiO₂.Al₂O₃.TiO₂, andTiO₂.CeO₂.SiO₂. Examples of desirable inorganic colloidal particles areSiO₂, Al₂O₃, TiO₂, ZrO₂, and Fe₂O₃. Silica colloidal particles(colloidal silica) are preferred. Since the surfaces of colloidalparticles are generally hydrophilic, they are suited to the preparationof colloidal solutions with water as the dispersion medium. For example,since the surface of colloidal silica obtained by common synthesismethods is covered with polarized oxygen atoms (O²⁻), it adsorbs waterwhen placed in water, forming hydroxyl groups and stabilizing. However,in the organic solvents that are commonly employed in magneticlayer-forming compositions, these particles tend not to remain presentin colloidal form. By contrast, the definition of the colloidalparticles in the present invention and present Specification isparticles that disperse without settling and that form colloidaldispersions when added in a quantity of 1 g per 100 mL of theabove-described organic solvents. Such colloidal particles can beprepared by known methods, such as by rendering the surface hydrophobicor the like by a surface treatment. Details describing such hydrophobictreatments are described, for example, in Japanese Unexamined PatentPublication (KOKAI) Heisei Nos. 5-269365 and 5-287213, and JapaneseUnexamined Patent Publication (KOKAI) No. 2007-63117. The contents ofthe above publications are expressly incorporated herein by reference intheir entirety.

Two manufacturing methods for desirable colloidal particles in the formof silica colloidal particles (colloidal silica) are generally known:the water glass method and the sol-gel method. The water glass method isa method employing a starting material in the form of sodium silicate(so-called water glass). This is ion-exchanged to generate activesilicic acid and grow particles. The sol-gel method is a method thatemploys tetraalkoxysilane as a starting material. As this is hydrolyzedin the presence of a basic catalyst, particles grow. When silicacolloidal particles are employed as the above nonmagnetic particles, thesilica colloidal particles can be manufactured by either of thesemanufacturing methods.

In the magnetic layer, the content of the nonmagnetic particles with aratio of major axis length to minor axis length (major axis length/minoraxis length) of less than or equal to 1.5 is desirably 0.1 to 10.0weight parts, preferably 0.1 to 5.0 weight parts, per 100.0 weight partsof ferromagnetic powder. In the present invention, a single type of agiven component can be incorporated, or two or more different structuresor materials can be incorporated. The content of a component that isincorporated in two or more types refers to the combined content of thecomponents. Multiple nonmagnetic particles with a ratio of major axislength to minor axis length (major axis length/minor axis length) ofless than or equal to 1.5 can be incorporated into the magnetic layer;the number thereof is not specifically limited.

(Additives)

The magnetic layer of the above magnetic recording medium contains thevarious components set forth above, and can contain one or more optionaladditives. An abrasive is an example of a desirable additive. Abrasivesare components that are capable of imparting the ability to removedeposits (abrasiveness) that adhere to the magnetic head during running.From the perspective of increasing the strength of the magnetic layer byadding an abrasive to the magnetic layer, particles that are anisotropicin shape are desirable as abrasives. In this regard, a nonmagneticparticle with a ratio of major axis length to minor axis length (majoraxis length/minor axis length) in excess of 1.5 is desirable, and anonmagnetic particle with a ratio of major axis length to minor axislength (major axis length/minor axis length) in excess of 1.5 but lessthan or equal to 5.0 is preferred as an abrasive. The ratio of majoraxis length to minor axis length (major axis length/minor axis length)of an abrasive is determined in a SEM image as set forth above fornonmagnetic particles with a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5. The major axis length of the various particles of abrasive, forexample, falls within a range of 30 nm to 100 nm, desirably within arange of 50 nm to 100 nm. Examples of abrasives are substances that arecommonly employed as abrasives in the magnetic layer, such as particlesof alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂, TiC,chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), ironoxide, and diamond. Of these, various particles of alumina such asα-alumina, silicon carbide, and diamond are desirable. The content ofabrasive in the magnetic layer desirably falls within a range of 1.0 to20.0 weight parts, preferably falls within a range of 3.0 to 15.0 weightparts, and more preferably, falls within a range of 4.0 to 10.0 weightparts, per 100.0 weight parts of ferromagnetic powder.

Examples of additives that can be incorporated into the magnetic layerare lubricants, dispersing agents, dispersion adjuvants, fungicides,antistatic agents, and oxidation inhibitors. Commercial products can besuitably selected for use as additives based on the properties desired.

The magnetic layer as set forth above can be disposed on a nonmagneticsupport either directly or over a nonmagnetic layer. Details regardingthe nonmagnetic layer and nonmagnetic support will be given furtherbelow.

<Nonmagnetic Layer>

In the above magnetic recording medium, the magnetic layer can bedisposed directly on a nonmagnetic support, or a nonmagnetic layercontaining nonmagnetic powder and binder can be present between thenonmagnetic support and the magnetic layer. The nonmagnetic powder thatis employed in the nonmagnetic layer can be an organic or an inorganicsubstance. Carbon black or the like can also be employed. Examples ofinorganic substances are metals, metal oxides, metal carbonates, metalsulfates, metal nitrides, metal carbides, and metal sulfides. Thesenonmagnetic powders are available as commercial products and can bemanufactured by known methods. Reference can be made to JapaneseUnexamined Patent Publication (KOKAI) No. 2011-216149, paragraphs 0146to 0150, for details. Reference can be made to Japanese UnexaminedPatent Publication (KOKAI) No. 2010-24113, paragraphs 0040 and 0041, fordetails on carbon black that can be used in the nonmagnetic layer. Thecontent (fill rate) of nonmagnetic powder in the nonmagnetic layerdesirably falls within a range of 50 to 90 weight %, preferably within arange of 60 to 90 weight %.

For other details regarding binder, additives, and the like in thenonmagnetic layer, known techniques relating to nonmagnetic layers canbe applied. As a further example, known techniques relating to magneticlayers can be applied with regard to the quantity and type of bindersand the quantity and type of additives.

The nonmagnetic layer of the magnetic recording medium in the presentinvention may be in the form of an essentially nonmagnetic layercontaining small quantities of ferromagnetic powder, either in the formof impurities or by intention, for example, along with nonmagneticpowder. In the present invention, the term “essentially nonmagneticlayer” refers to a layer with a residual magnetic flux density of lessthan or equal to 10 mT or a coercive force of less than or equal to 7.96kA/m (100 Oe), or a layer with a residual magnetic flux density of lessthan or equal to 10 mT and a coercive force of less than or equal to7.96 kA/m (100 Oe). The nonmagnetic layer desirably has neither residualmagnetic flux density nor coercive force.

<Backcoat Layer>

In the above magnetic recording medium, a backcoat layer containingnonmagnetic powder and binder can be present on the opposite side of thenonmagnetic support from the side on which the magnetic layer ispresent. Carbon black and/or inorganic powder are desirably incorporatedinto the backcoat layer. Known techniques relating to the formulas ofthe magnetic layer and the nonmagnetic layer can be applied to thebinder that is contained, and the various additives that can beoptionally contained, in the backcoat layer.

<Nonmagnetic Support>

The nonmagnetic support will be described next. Known nonmagneticsupports in the form of biaxially stretched polyethylene terephthalate,polyethylene naphthalate, polyamide, polyamide-imide, aromaticpolyamide, and the like are examples. Of these, polyethyleneterephthalate, polyethylene naphthalate, and polyamide are desirable.These supports can be subjected in advance to treatments such as coronadischarge, plasma treatments, adhesion-enhancing treatments, and heattreatments.

<Thicknesses of Nonmagnetic Support and Various Layers>

The thickness of the nonmagnetic support and various layers in the abovemagnetic recording medium are as follows. The thickness of thenonmagnetic support is, for example, 3.00 μm to 80.00 μm, desirably 3.00μm to 50.00 μm, and preferably 3.00 μm to 10.00 μm.

From the perspective of achieving higher density recording, thethickness of the magnetic layer is desirably less than or equal to 100nm. The thickness of the magnetic layer can be determined by the methodset forth above. The thickness of other layers and the nonmagneticsupport can be determined by the same method as the thickness of themagnetic layer, or can be determined by a known film thicknessmeasurement method. It can also be determined as a design thickness bycalculation from the manufacturing conditions, such as the coatingconditions. The thickness of the magnetic layer preferably falls withina range of from 10 nm to 100 nm, and more preferably, within a range of20 nm to 90 nm. The magnetic layer can be in the form of at least onelayer, or can be divided into two or more layers having differentmagnetic characteristics. Configurations known for multilayered magneticlayers can be applied.

The thickness of the nonmagnetic layer desirably falls within a range of0.01 μm to 0.60 μm, preferably within a range of 0.05 μm to 0.20 μm, andmore preferably, within a range of 0.10 μm to 0.20 μm.

The thickness of the backcoat layer is desirably less than or equal to0.90 μm, preferably 0.10 μm to 0.70 μm.

<Manufacturing Method>

(Preparation of Compositions for Forming Various Layers)

The compositions for forming the magnetic layer, optionally providednonmagnetic layer, and backcoat layer normally contain solvent inaddition to the various components set forth above. Examples of solventare the various organic solvents that are commonly employed tomanufacturing particulate magnetic recording media. Specific examplesare, in any proportions, ketones such as acetone, methyl ethyl ketone,methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone,and tetrahydrofuran; alcohols such as methanol, ethanol, propanol,butanol, isobutyl alcohol, isopropyl alcohol, and methyl cyclohexanol;esters such as methyl acetate, butyl acetate, isobutyl acetate,isopropyl acetate, ethyl lactate, and glycol acetate; glycol ethers suchas glycol dimethyl ether, glycol monoethyl ether, and dioxane; aromatichydrocarbons such as benzene, toluene, xylene, cresol, andchlorobenzene; chlorinated hydrocarbons such as methylene chloride,ethylene chloride, carbon tetrachloride, chloroform, ethylenechlorohydrin, and dichlorobenzene; N,N-dimethyl formamide; and hexane.

The process of preparing the compositions for forming the variouslayers, such as the magnetic layer, optionally provided nonmagneticlayer and backcoat layer, normally includes at least a kneading step, adispersion step, and mixing steps provided before and after these stepsas needed. Each of these steps can be divided into two or more stages.All of the starting materials in the form of ferromagnetic powder,binder, nonmagnetic particles with a ratio of the major axis length tothe minor axis length (major axis length/minor axis length) of less thanor equal to 1.5, various additives, solvent and the like that areemployed in the present invention can be added at the start, or part waythrough, any of these steps. An individual starting material can bedivided for addition in two or more steps. For example, binder can bedivided up and added in the kneading step, dispersing step, and in amixing step after the dispersing step for viscosity adjustment. Tomanufacture the above magnetic recording medium, conventionally knownmanufacturing techniques can be employed. An open kneader, continuouskneader, pressurized kneader, extruder, or some other device withpowerful kneading force is desirably employed in the kneading step.Details regarding these kneading processes are given in JapaneseUnexamined Patent Publication (KOKAI) Heisei Nos. 1-106338 and 1-79274,which are expressly incorporated herein by reference in their entirety.Glass beads or some other form of bead can be employed to disperse thecompositions for forming the various layers. High-density dispersionbeads in the form of zirconia beads, titania beads, and steel beads aresuitable as such dispersion beads. The particle diameter and fill rateof these dispersion beads can be optimized for use. A known dispersercan be employed.

(Coating Step)

The magnetic layer can be formed by coating the magnetic layer-formingcomposition directly on a nonmagnetic support, or sequentially orsimultaneously multilayer coating the magnetic layer-forming compositionand the nonmagnetic layer-forming composition. The backcoat layer can beformed by coating the backcoat layer-forming composition on the oppositeside of the nonmagnetic support from the side on which the magneticlayer is present (or on the side on which a magnetic layer is providedlater). For details regarding the coatings to form the various layers,reference can be made to paragraph 0066 of Japanese Unexamined PatentPublication (KOKAI) No. 2010-231843, which is expressly incorporatedherein by reference in its entirety.

(Other Steps)

For various other steps in manufacturing the magnetic recording medium,reference can be made to paragraphs 0067 to 0070 of Japanese UnexaminedPatent Publication (KOKAI) No. 2010-231843.

An Embodiment of Desirable Manufacturing Method

An example of a desirable manufacturing method is the followingmanufacturing method that can localize nonmagnetic particles with aratio of major axis length to minor axis length (major axis length/minoraxis length) of less than or equal to 1.5 on the magnetic layer sidesurface of the magnetic recording medium:

A method of manufacturing the above magnetic recording medium, includingformation of a magnetic layer,

wherein the formation of a magnetic layer includes:

coating and drying a magnetic coating film-forming compositioncontaining ferromagnetic powder and binder, but not containingnonmagnetic particles with a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5, on a nonmagnetic support, either directly or over one or more otherlayers (such as the nonmagnetic layer) to form a magnetic coating film;

coating and drying an overcoat liquid, containing multiple nonmagneticparticles having a ratio of major axis length to minor axis length(major axis length/minor axis length) of less than or equal to 1.5 andsolvent, on the above magnetic coating film to dispose the nonmagneticparticles on the above magnetic coating film; and

pressing into the magnetic coating film the nonmagnetic particles thathave been disposed to form a magnetic layer that contains ferromagneticpowder and binder and in which the above nonmagnetic particles arepresent in the above-described state.

The above method of manufacturing the above magnetic recording mediumwill be described in greater detail below.

The method of manufacturing the above magnetic recording medium can beimplemented in the same manner as the conventional method ofmanufacturing a particulate magnetic recording medium with the exceptionof conducting the step of forming a magnetic coating film not containingnonmagnetic particles having a ratio of major axis length to minor axislength (major axis length/minor axis length) of less than or equal to1.5 through the step of pressing in the nonmagnetic particles in themagnetic layer forming process.

The magnetic coating film can be formed in the same manner as whenforming a common magnetic layer with the exception that the compositionemployed to form the coating film does not contain nonmagnetic particleshaving a ratio of major axis length to minor axis length (major axislength/minor axis length) of less than or equal to 1.5. For example, theabove magnetic coating film can be formed by directly coating thecomposition for forming the coating film, or sequentially orsimultaneously multilayer coating it with a nonmagnetic layer-formingcomposition, on a nonmagnetic support.

The overcoat liquid that is coated over the magnetic coating film thathas been formed contains solvent and multiple nonmagnetic particleshaving a ratio of major axis length to minor axis length (major axislength/minor axis length) of less than or equal to 1.5. The solventemployed can be any of the various organic solvents set forth above.When the nonmagnetic particles are colloidal particles, it is desirableto employ solvent that is capable of stably dispersing colloidalparticles. In the present invention and present specification, the term“colloidal particle” refers, as set forth above, to particles that arecapable of dispersing without settling to form a colloidal dispersionwhen 1 g is added per 100 mL of an organic solvent comprised of at leastmethyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or amixed solvent containing two or more of these solvents in any mixingratio. Thus, the solvent selected from the above solvents is desirablyused to prepare an overcoat liquid containing the colloidal particles.The concentration of the nonmagnetic particles in the overcoat liquidcan be, for example, 5 to 50 weight %, but there is no limitation tothis range. In addition to the above nonmagnetic particles and solvent,the overcoat liquid can contain a lubricant in any quantity.

The overcoat liquid can be coated and dried in the same manner as (orbased on) the usual magnetic layer-forming composition. Thus thenonmagnetic particles can be disposed on the magnetic coating film.

The nonmagnetic particles that have been disposed on the magneticcoating film can be pressed in on the magnetic coating film side to forma magnetic layer that contains ferromagnetic powder and binder in whichthe nonmagnetic particles are present in a state where the average valueof the ratio of b/t is less than or equal to 0.9. A known method can beused to press in the nonmagnetic particles, such as a surface smoothingtreatment of the magnetic recording medium by calender processing or thelike. The degree of pressing in can be controlled by means of thecalender pressure or the like to adjust the average value of the ratioof b/t.

An embodiment of a desirable manufacturing method has been set forthabove. However, the magnetic recording medium according to an aspect ofthe present invention is not limited to being manufactured by the abovemanufacturing method.

Magnetic Signal Reproduction Device

An aspect of the present invention relates to a magnetic signalreproduction device including the above magnetic recording medium and amagnetic reproduction head.

So long as the magnetic signal reproduction device includes the magneticrecording medium according to an aspect of the present invention, knowntechniques can be applied without limitation to the remaining mattersthereof.

In one desirable embodiment, a magnetic signal that has been recorded ata linear recording density of greater than or equal to 250 kfci on theabove magnetic recording medium is reproduced by a magnetic reproductionhead in the above magnetic signal reproduction device. The magneticrecording medium according to an aspect of the present invention can besuitably used to record and reproduce a magnetic signal at a linearrecording density of greater than or equal to 200 kfci, or greater thanor equal to 250 kfci. The linear recording density is, for example, lessthan or equal to 800 kfci, but may also exceed 800 kfci.

In recording and reproducing magnetic signals, the protrusions that arepresent on the magnetic layer side surface of the magnetic recordingmedium increase the spacing between the magnetic head and the surface ofthe magnetic recording medium during recording and reproduction. In thisregard, the output drop Ls due to the increased spacing is known to berelated as follows, where d=spacing [nm] between the magnetic head andthe surface of the magnetic recording medium and λ=recording wavelength[nm].

Ls[dB]=54.6(d/λ)

As will be understood from the above relation, the shorter the recordingwavelength, the more it is possible to inhibit a drop in output byreducing the spacing. Short wavelength recording means the same thing ashigh-density recording. Thus, the higher the recording density, the moreit becomes possible to inhibit a drop in output by reducing the spacing.In this regard, the present inventor presumes that having numerousprotrusions greater than or equal to 20 nm in height causes a drop inoutput due to spacing in the high-density recording region.Additionally, the present inventor presumes that keeping the number ofprotrusions greater than or equal to 20 nm in height to within the rangeset forth above and causing protrusions greater than or equal to 5 nm inheight to be present in the number set forth above make it possible toboth achieve better electromagnetic characteristics and reduce thecoefficient of friction during running even in the high-densityrecording range.

For details regarding the structure and the like of the magnetic signalreproduction device, reference can be made to paragraphs 0072 and 0073of Japanese Unexamined Patent Publication (KOKAI) No. 2010-231843.

EXAMPLES

The present invention will be described based on Examples below.However, the present invention is not intended to be limited by theembodiments described in Examples. The “parts” and “%” indicated belowdenote “weight parts” and “weight %” unless specifically indicatedotherwise

The colloidal particles (colloidal silica) employed in Examples 1 to 5,7 and Comparative Example 3 set forth below are all commercial silicacolloidal particles prepared by the sol-gel method. Table 1 below givesthe average particle size (average major axis length) as measured by themethod used to measure the average particle size of various powders, thestandard deviation, the coefficient of variation in particle size (CVvalue), and the average acicular ratio. These silica colloidal particleshave properties corresponding to the definition of colloidal particlesset forth above.

The silica colloidal particles (colloidal silica) employed inComparative Examples 1 and 4 are commercial silica colloidal particlesprepared by the water glass method. Table 1 below gives the averageparticle size (average major axis length) as measured by the method usedto measure the average particle size of various powders, the standarddeviation, the coefficient of variation in particle size (CV value), andthe average acicular ratio. These silica colloidal particles haveproperties corresponding to the definition of colloidal particles setforth above.

The silicon oxide particles employed in Example 6 and ComparativeExample 5 are silicon oxide particles (powdered silica) that do notcorrespond to the definition of colloidal particles set forth above.Table 1 below gives the average particle size (average major axislength) as measured by the method used to measure the average particlesize of various powders, the standard deviation, the coefficient ofvariation in particle size (CV value), and the average acicular ratio.

The carbon black employed in Comparative Example 2 is commercial carbonblack that does not correspond to the definition of colloidal particlesset forth above. Table 1 below gives the average particle size (averagemajor axis length) as measured by the method used to measure the averageparticle size of various powders, the standard deviation, thecoefficient of variation in particle size (CV value), and the averageacicular ratio.

TABLE 1 Average particle size (Average coefficient major of Nonmagneticparticles axis variation Average Contained length) Standard (CV value)acicular in the magnetic layer [nm] deviation [%] ratio Colloidal silicaA 45 5.6 12.4 1.1 (employed in Examples 1 to 3, 7 and Comp. Ex. 3)Colloidal silica B 25 3.8 15.2 1.4 (employed in Examples 4, 5) Colloidalsilica C 200 11.0 5.5 1.1 (employed in Comp. Ex. 1 ) Colloidal silica D110 14.0 12.7 1.1 (employed in Comp. Ex. 4) Silicon oxide particles a 505.6 11.2 1.5 (employed in Example 6) Silicon oxide particles b 50 18.036.0 1.8 (employed in Comp. Ex. 5) Carbon black 45 60.0 133.3 1.9(employed in Comp. Ex. 2)

Example 1 (1) Formula of Magnetic Coating Film-Forming Composition

(Magnetic liquid) Ferromagnetic hexagonal barium ferrite powder: 100.0parts (Coercive force Hc: 175 kA/m (2,200 Oe), average particle size(average plate diameter): 20 nm) Oleic acid: 2.0 parts Vinyl chloridecopolymer (MR-104 made 10.0 parts by Zeon Corp.): Sulfonic acidgroup-containing polyester 4.0 parts polyurethane resin (UR-4800 made byToyobo): Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts(Abrasive liquid) Diamond particles (average particle size: 6.0 parts 50nm, average acicular ratio: 4.0): Sulfonic acid group-containingpolyester 0.6 part polyurethane resin (UR-4800 made by Toyobo):Cyclohexanone: 23.0 parts (Lubricant and curing agent solution) Stearicacid: 2.0 parts Amide stearate: 0.3 part Butyl stearate: 6.0 partsMethyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 partsPolyisocyanate (Coronate L made by 3.0 parts Nippon PolyurethaneIndustry Co., Ltd.):

(2) Formula of Nonmagnetic Layer-Forming Composition

Red iron oxide (average particle size: 0.15 μm, 75.0 parts averageacicular ratio: 7, Brunauer-Emmett- Teller (BET) specific surface area:52 m²/g): Carbon black (average particle size: 16 nm, 25.0 parts dibutylphthalate (DBP) oil absorption capacity: 74 cm³/100 g): Phenylphosphonicacid: 3.0 parts Vinyl chloride copolymer (MR-104 from Zeon Corp.): 12.0parts Sulfonic acid group-containing polyester 8.0 parts polyurethaneresin (UR-8401) made by Toyobo: Methyl ethyl ketone: 370.0 partsCyclohexanone: 370.0 parts Stearic acid: 1.0 parts Amide stearate: 0.3part Butyl stearate: 2.0 parts Polyisocyanate (Coronate L made by Nippon5.0 parts Polyurethane Industry Co., Ltd.):

(3) Formula of Backcoat Layer-Forming Composition

Carbon black (average particle size: 40 nm, 100.0 parts DBP oilabsorption capacity: 74 cm³/100 g): Copper phthalocyanine: 3.0 partsNitrocellulose: 25.0 parts Sulfonic acid group-containing polyester 60.0parts polyurethane resin (UR-8401) made by Toyobo: Polyester resin(Vylon 500 made by Toyobo): 4.0 parts Alumina powder (α-alumina with BET1.0 part specific surface area of 17 m²/g): Polyisocyanate (Coronate Lmade by Nippon 15.0 parts Polyurethane Industry Co., Ltd.): Methyl ethylketone: 600.0 parts Toluene: 600.0 parts

(4) Preparation of Various Layer-Forming Compositions

The magnetic coating film-forming composition was prepared by thefollowing method.

After the above magnetic liquid was kneaded and diluted in an openkneader, zirconia (ZrO₂) beads 0.1 mm in diameter (referred to as “Zrbeads” hereinafter) were employed to conduct 30 passes of dispersionprocessing, each pass including a retention time of 2 minutes, at a beadfill rate of 80 volume % and a rotor tip peripheral speed of 10 m/s in ahorizontal bead mill disperser.

The abrasive liquid was prepared by charging the above components, alongwith Zr beads 0.3 mm in diameter, to a horizontal bead mill disperserand adjusting them so that the (bead volume/(abrasive liquid volume+beadvolume))×100 was 80%; conducting bead mill dispersion processing for 120minutes; removing the liquid following processing; and employing aflow-type ultrasonic dispersion filtration device to conduct ultrasonicdispersion filtration processing.

The magnetic liquid, abrasive liquid, and lubricant and curing agentsolution were charged to a dissolver-stirrer and stirred for 30 minutesat a peripheral speed of 10 m/s. The mixture was then processed for 3passes at a flow rate of 7.5 kg/min in a flow-type ultrasonic dispersionfiltration device and filtered through a filter having an average porediameter of 1 μm to prepare a magnetic coating film-forming composition.

The nonmagnetic layer-forming coating composition was prepared by thefollowing method.

With the exception of the lubricants (stearic acid, amide stearate, andbutyl stearate) and the polyisocyanate, the above components werekneaded and diluted in an open kneader. Subsequently, dispersionprocessing was conducted in a horizontal bead mill disperser. Thelubricants and polyisocyanate were then added, and a dissolver-stirrerwas used to stir and mix the mixture to prepare a nonmagneticlayer-forming composition.

The backcoat layer-forming composition was prepared by the followingmethod.

With the exception of the polyisocyanate, the above components werecharged to a dissolver-stirrer and stirred for 30 minutes at aperipheral speed of 10 m/s. The mixture was then subjected to adispersion treatment in a horizontal bead mill disperser. Subsequently,the polyisocyanate was added and the mixture was stirred and mixed in adissolver-stirrer to prepare a backcoat layer-forming composition.

(5) Preparation of Magnetic Tape (Forming the Nonmagnetic Layer, BackCoat Layer, and Magnetic Coating Film)

The nonmagnetic layer-forming composition was coated and dried to a drythickness of 0.10 μm on a polyethylene naphthalate support 6.00 μm inthickness. The backcoat layer-forming composition was then coated anddried to a dry thickness of 0.50 μm on the surface on the opposite sideof the support from that on which the nonmagnetic layer had been formed.The support on which the nonmagnetic layer and backcoat layer had beenformed was rolled up and heat treated for 36 hours in an environmentwith an atmospheric temperature of 70° C. Following the heat treatment,the magnetic coating film-forming composition was coated and dried onthe nonmagnetic layer to form a magnetic coating film.

Subsequently, a surface smoothing treatment (calendering treatment) wasconducted at a calender roll surface temperature of 100° C., a linearpressure of 300 kg/cm (294 kN/m), and a rate of 40 m/min with calenderrolls comprised solely of metal rolls.

(Preparation of Overcoat Liquid)

An overcoat liquid was prepared by dispersing 20.0 parts of silicacolloidal particles (colloidal silica A) in 80.0 parts of methyl ethylketone.

(Forming the Magnetic Layer)

The coating quantity of the overcoat liquid that had been formed wasadjusted so that the quantity of silica colloidal particles per 100.0parts of ferromagnetic hexagonal barium ferrite powder contained in themagnetic coating film was the quantity indicated in Table 2 below andthen coated and dried on the magnetic coating film to dispose silicacolloidal particles on the magnetic coating film.

Subsequently, a calendering treatment was conducted at a calender rollsurface temperature of 100° C., a linear pressure of 300 kg/cm (294kN/m), and a speed of 40 m/min with calender rolls comprised solely ofmetal rolls to press in the silica colloidal particles that had beendisposed on the magnetic coating film and form a magnetic layer.

Subsequently, a heat treatment was conducted for 36 hours in anenvironment of an atmospheric temperature of 70° C. Following the heattreatment, the product was slit to ½ inch (0.0127 meter) width.Following slitting, a tape cleaning device on which were mounted asapphire blade and nonwoven cloth pressing against the surface of themagnetic layer was used to clean the surface of the magnetic layer andobtain a magnetic recording medium (magnetic tape).

Examples 2 to 7 and Comparative Examples 2 and 5

With the exception that the quantities of the various nonmagneticparticles per 100.0 parts of the ferromagnetic hexagonal barium ferritepowder contained in the magnetic coating film indicated in Table 1 wereadjusted to the quantities given in Table 2 below and coated and dried,magnetic recording media (magnetic tapes) were obtained by the samemethod as in Example 1. To thicken the magnetic coating film that wasformed in Example 7, the quantity of the magnetic coating film-formingcomposition that was coated was increased relative to Example 1.

Comparative Examples 1, 3, 4

With the exception that, to the magnetic liquid, the various nonmagneticparticles indicated in Table 1 were added to achieve the quantitiesgiven in Table 2 below per 100.0 parts of the ferromagnetic hexagonalbarium ferrite powder contained in the magnetic liquid, and that thesteps from coating of the overcoat liquid to the calendering treatmentwere not implemented, magnetic recording media (magnetic tapes) wereobtained by the same method as in Example 1.

[Evaluation Methods]

1. Average Value of Ratio of b/t

SEM images of the magnetic tapes of Examples and Comparative Exampleswere picked up by the method set forth above and the magnetic layerthickness t was determined by the method set forth above. In the SEMimage that was picked up, all of the nonmagnetic particles with a ratioof major axis length to minor axis length (major axis length/minor axislength) of less than or equal to 1.5 were specified and the ratio of b/twas determined by the method set forth above.

In Comparative Examples 2 and 5, no nonmagnetic particles with a ratioof major axis length to minor axis length (major axis length/minor axislength) of less than or equal to 1.5 were observed. Instead, the ratioof b/t was determined by the method set forth above for carbon blackspecified in the SEM image that was picked up in Comparative Example 2,and for silicon oxide particles specified in the SEM image that waspicked up in Comparative Example 5.

In the above Examples and Comparative Examples, nonmagnetic particles inthe form of diamond particles that were contained in the magnetic layeralong with the nonmagnetic particles specified above had an averageacicular ratio of 4.0 as set forth above. The shape differed greatlyfrom that of the nonmagnetic particles specified above. Accordingly, inthe SEM image, the diamond particles were readily distinguishable fromthe nonmagnetic particles specified above. However, it was also possibleto specify the various particles by the known methods of compositionalanalysis set forth above.

In the magnetic tapes of Examples and Comparative Examples, when theratio of diamond particles (major axis length/minor axis length) in themagnetic layer was determined by the method set forth above, the ratio(major axis length/minor axis length) of all of the diamond particlescontained in the magnetic layer, at about 4.0, greatly exceeded 1.5.

Samples for sectional observation were prepared by the following method.

(i) Samples measuring 10 mm in the width direction of the magnetictape×10 mm in the thickness direction were cut out with a razor.

A protective film was formed on the surface of the magnetic layer of thesamples that were cut out to obtain samples with protective films. Theprotective films were formed by the following method.

A platinum (Pt) film (30 nm thick) was formed by sputtering on thesurface of the magnetic layer of the sample. The platinum filmsputtering was conducted under the following conditions.

<Platinum Film Sputtering Conditions> Target: Pt

Degree of vacuum within chamber of sputtering device: less than or equalto 7 Pa

Current: 15 mA

A carbon film was further formed to a thickness of 100 nm to 150 nm onthe samples equipped with platinum films that were prepared above. Thecarbon film was formed by a chemical vapor deposition (CVD) mechanismwith a gallium ion (Ga⁺) beam installed on the focused ion beam (FIB)employed in (ii) below.

(ii) The samples with protective films prepared in (i) above were FIBprocessed with a gallium ion (Ga⁺) beam using an FIB device to exposethe cross-section of the magnetic tape. The acceleration voltage in theFIB processing was 30 kV and the probe current was 1,300 pA.

The samples for sectional observation that had been thus exposed wereobserved by SEM to determine the average value of the ratio of b/t.

2. Average Major Axis Length φ, φ/t

The arithmetic average (average major axis length) φ of the major axislength of the nonmagnetic particles with a ratio of major axis length tominor axis length (major axis length/minor axis length) of less than orequal to 1.5 that were specified in 1, above, was calculated, and theratio with the thickness of the magnetic layer determined in 1, above,φ/t, was determined for Examples 1 to 7 and Comparative Examples 1, 3,and 4.

The arithmetic average (average major axis length) φ of the major axislength was calculated and the ratio with the thickness of the magneticlayer determined in 1, above, φ/t, was determined for the carbon blackspecified in 1, above, for Comparative Example 2 and for the siliconoxide particles specified in 1, above, for Comparative Example 5.

When the coefficient of variation (CV value) was calculated from theaverage major axis length φ obtained and the standard deviation σ of themajor axis length, values identical to the CV values indicated in Table1 were obtained for the various nonmagnetic particles employed inExamples and Comparative Examples.

3. Number of Protrusions Measured by AFM on the Magnetic Layer SideSurface of the Magnetic Tape

The number of protrusions greater than or equal to 5 nm in height andthe number of protrusions greater than or equal to 20 nm in height weredetermined by the method set forth above for the magnetic tapes ofExamples and Comparative Examples.

4. Evaluation of Electromagnetic Characteristics

The SNR was measured for each magnetic tape of Examples and ComparativeExamples with a ½ inch (0.0127 meter) reel tester on which had beensecured a magnetic head. The relative speed of the magnetichead/magnetic tape was 5.5 m/sec. A metal-in-gap (MIG) head (gap length:0.15 μm, track width: 1.0 μm) was employed for recording. The recordingcurrent was set to the optimal recording current of each magnetic tape.The reproduction head employed was a giant magnetoresistive (GMR) headwith an element thickness of 15 nm, a shield spacing of 0.1 μm, and aread width of 0.5 μm. A signal was recorded at a linear recordingdensity of 540 kfci. The reproduced signal was measured with a spectrumanalyzer made by Shibasoku. The ratio of the carrier output to theintegrated noise of the full spectrum bandwidth was adopted as the SNR.Based on the SNR obtained, the electromagnetic characteristics wereevaluated on the scale below. An evaluation result of A indicates adetermination that a higher level of improvement was achieved in theelectromagnetic characteristics.

Adopting the SNR of Comparative Example 1 as 0.0 dB:

SNR in excess of +0.5 dB: ASNR greater than or equal to −0.5 dB but less than or equal to +0.5 dB:BSNR less than −0.5 dB: C

5. Measurement of Coefficient of Friction

Each of the tapes of Examples and Comparative Examples was wound on around rod made of AlTiC (alumina titanium carbide) with a centerlineaverage surface roughness Ra as measured for a 40 μm square (40 μm×40μm) by AFM of 15 nm and a diameter of 4 mm such that the width directionof the magnetic tape was parallel to the axial direction of the roundrod. A 100 g weight was hung from one end of the magnetic tape and theother end was slid 45 mm per pass at a speed of 14 mm/sec while mountedon a load cell. The sliding was repeated for a total of 100 passes. Theload during sliding at identical speed was detected by the load cell atpass 1 and pass 100 and measurement values were obtained. Thecoefficient of friction was calculated based on the following equation:

Coefficient of friction=ln(measured value(g)/100(g))/π

The coefficients of friction at pass 1 and pass 100 were evaluated onthe following scale:Less than 0.30: AGreater than or equal to 0.30 but less than 0.40: BGreater than or equal to 0.40 but less than 0.50: CGreater than or equal to 0.50 or occurrence of sticking precludingmeasurement: D

An evaluation result of A or B resulted in a determination that adecrease in the coefficient of friction during running had beenachieved.

The results of the above are given in Table 2.

TABLE 2 Nonmagnetic particles indicated in Table 1 and conotained in themagnetic layer Thickness Average major of the Method for Average axislength/ magnetic adding to the Content major Thickness of the layermagnetic [weight axis length magnetic layer t[nm] Type layer parts] φ[nm] φ/t Ex. 1 50 Colloidal silica A Overcoat (OC) 1.7 45 0.9 Ex. 2 50Colloidal silica A OC 3.5 45 0.9 Ex. 3 50 Colloidal silica A OC 0.5 450.9 Ex. 4 50 Colloidal silica B OC 0.2 25 0.5 Ex. 5 50 Colloidal silicaB OC 0.6 25 0.5 Ex. 6 50 Silicon oxide OC 1.7 50 1.0 particles a Ex. 7100 Colloidal silica A OC 1.7 45 0.5 Comp. 50 Colloidal silica C Mixedin the 3.5 200 4.0 Ex. 1 magnetic liquid Comp. 50 Carbon black OC 1.7 450.9 Ex. 2 Comp. 50 Colloidal silica A Mixed in the 1.7 45 0.9 Ex. 3magnetic liquid Comp. 50 Colloidal silica D Mixed in the 1.7 110 2.2 Ex.4 magnetic liquid Comp. 50 Silicon oxide OC 1.7 50 1.0 Ex. 5 particles bNonmagnetic particles indicated in Table 1 and conotained in Number ofprotrusions the magnetic layer greater than greater than Average orequal to or equal to value of 5 nm in height 20 nm in height Evaluationresults the ratio [per an area [per an area Coefficients of friction SNRb/t 40 μm × 40 μm] 40 μm × 40 μm] Pass 1 Pass 100 [dB] Ex. 1 0.8 3200 3B: 0.35 B: 0.39 A: 1.5   Ex. 2 0.8 9500 2 A: 0.25 A: 0.25 A: +1.5 Ex. 30.8 850 0 B: 0.39 B: 0.39 A: +2.2 Ex. 4 0.4 3140 0 B: 0.37 B: 0.39 A:+1.8 Ex. 5 0.4 9500 0 A: 0.25 A: 0.25 A: +1.8 Ex. 6 0.9 3150 3 B: 0.35B: 0.39 A: +1.5 Ex. 7 0.4 3230 3 B: 0.38 B: 0.39 A: +1.2 Comp. 1.0 195636 A: 0.28 A: 0.20 0 Ex. 1 Comp. 0.8 5689 50 A: 0.22 A: 0.25 C: −1.5 Ex.2 Comp. 0.9 760 0 D: sticking Measurement Measurement Ex. 3 was wasimpossible. impossible. Comp. 1.0 3100 25 A: 0.25 A: 0.20 B: +0.2 Ex. 4Comp. 0.9 2950 30 A: 0.25 A: 0.20 B: 0     Ex. 5

Based on the results in Table 2, the magnetic tapes of Examples werefound to afford a low coefficient of friction during running and goodelectromagnetic characteristics.

An aspect of the present invention can be useful in the technical fieldof magnetic recording media for high-density recording, such as backuptapes.

Although the present invention has been described in considerable detailwith regard to certain versions thereof, other versions are possible,and alterations, permutations and equivalents of the version shown willbecome apparent to those skilled in the art upon a reading of thespecification and study of the drawings. Also, the various features ofthe versions herein can be combined in various ways to provideadditional versions of the present invention. Furthermore, certainterminology has been used for the purposes of descriptive clarity, andnot to limit the present invention. Therefore, any appended claimsshould not be limited to the description of the preferred versionscontained herein and should include all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

Having now fully described this invention, it will be understood tothose of ordinary skill in the art that the methods of the presentinvention can be carried out with a wide and equivalent range ofconditions, formulations, and other parameters without departing fromthe scope of the invention or any Examples thereof.

All patents and publications cited herein are hereby fully incorporatedby reference in their entirety. The citation of any publication is forits disclosure prior to the filing date and should not be construed asan admission that such publication is prior art or that the presentinvention is not entitled to antedate such publication by virtue ofprior invention.

What is claimed is:
 1. A magnetic recording medium, which comprises amagnetic layer comprising ferromagnetic powder and binder on anonmagnetic support; wherein the magnetic layer comprises multiplenonmagnetic particles having a ratio of a major axis length to a minoraxis length, major axis length/minor axis length, of less than or equalto 1.5; the multiple nonmagnetic particles are present in the magneticlayer in a state where, when a depth to which each of the multiplenonmagnetic particles is embedded in the magnetic layer in observationof a sectional image picked up by a scanning electron microscope isdenoted as b and a thickness of the magnetic layer is denoted as t, anaverage value of a ratio of b/t is less than or equal to 0.9; and anumber of protrusions greater than or equal to 5 nm in height is greaterthan or equal to 800 and a number of protrusions greater than or equalto 20 nm in height is less than or equal to 20 as measured by an atomicforce microscope per an area 40 μm×40 μm on a magnetic layer sidesurface of the magnetic recording medium.
 2. The magnetic recordingmedium according to claim 1, wherein a ratio φ/t of an average majoraxis length φ of the multiple nonmagnetic particles having a ratio of amajor axis length to a minor axis length, major axis length/minor axislength, of less than or equal to 1.5 that are contained in the magneticlayer to the thickness of the magnetic layer is less than or equal to1.0.
 3. The magnetic recording medium according to claim 1, wherein theaverage major axis length φ of the multiple nonmagnetic particles havinga ratio of a major axis length to a minor axis length, major axislength/minor axis length, of less than or equal to 1.5 that arecontained in the magnetic layer ranges from 10 nm to 100 nm.
 4. Themagnetic recording medium according to claim 1, wherein the thickness ofthe magnetic layer t is less than or equal to 100 nm.
 5. The magneticrecording medium according to claim 1, wherein the multiple nonmagneticparticles having a ratio of a major axis length to a minor axis length,major axis length/minor axis length, of less than or equal to 1.5 thatare contained in the magnetic layer are inorganic oxide particles. 6.The magnetic recording medium according to claim 5, wherein theinorganic oxide particles are silicon oxide particles.
 7. The magneticrecording medium according to claim 6, wherein the silicon oxideparticles are silica colloidal particles.
 8. The magnetic recordingmedium according to claim 2, wherein the average major axis length φ ofthe multiple nonmagnetic particles having a ratio of a major axis lengthto a minor axis length, major axis length/minor axis length, of lessthan or equal to 1.5 that are contained in the magnetic layer rangesfrom 10 nm to 100 nm.
 9. The magnetic recording medium according toclaim 8, wherein the thickness of the magnetic layer t is less than orequal to 100 nm.
 10. The magnetic recording medium according to claim 9,wherein the multiple nonmagnetic particles having a ratio of a majoraxis length to a minor axis length, major axis length/minor axis length,of less than or equal to 1.5 that are contained in the magnetic layerare inorganic oxide particles.
 11. The magnetic recording mediumaccording to claim 10, wherein the inorganic oxide particles are siliconoxide particles.
 12. The magnetic recording medium according to claim11, wherein the silicon oxide particles are silica colloidal particles.13. The magnetic recording medium according to claim 1, wherein thenumber of protrusions greater than or equal to 5 nm in height asmeasured by an atomic force microscope per an area of 40 μm×40 μm on themagnetic layer side surface of the magnetic recording medium is greaterthan or equal to 800 but less than or equal to 500,000.
 14. The magneticrecording medium according to claim 1, wherein the number of protrusionsgreater than or equal to 5 nm in height as measured by an atomic forcemicroscope per an area of 40 μm×40 μm on the magnetic layer side surfaceof the magnetic recording medium is greater than or equal to 4,000. 15.The magnetic recording medium according to claim 1, wherein the numberof protrusions greater than or equal to 20 nm in height as measured byan atomic force microscope per an area of 40 μm×40 μm on the magneticlayer side surface of the magnetic recording medium is less than orequal to
 10. 16. The magnetic recording medium according to claim 1,which comprises a nonmagnetic layer comprising nonmagnetic powder andbinder between the nonmagnetic support and the magnetic layer.
 17. Amagnetic signal reproduction device, which comprises a magneticrecording medium and a magnetic reproduction head; wherein the magneticrecording medium comprises a magnetic layer comprising ferromagneticpowder and binder on a nonmagnetic support; wherein the magnetic layercomprises multiple nonmagnetic particles having a ratio of a major axislength to a minor axis length, major axis length/minor axis length, ofless than or equal to 1.5; the multiple nonmagnetic particles arepresent in the magnetic layer in a state where, when a depth to whicheach of the multiple nonmagnetic particles is embedded in the magneticlayer in observation of a sectional image picked up by a scanningelectron microscope is denoted as b and a thickness of the magneticlayer is denoted as t, an average value of a ratio of b/t is less thanor equal to 0.9; and a number of protrusions greater than or equal to 5nm in height is greater than or equal to 800 and a number of protrusionsgreater than or equal to 20 nm in height is less than or equal to 20 asmeasured by an atomic force microscope per an area 40 μm×40 μm on amagnetic layer side surface of the magnetic recording medium.
 18. Themagnetic signal reproduction device according to claim 17, whichreproduces with the magnetic reproduction head a magnetic signal thathas been recorded at a linear recording density of greater than or equalto 200 kfci on the magnetic recording medium.
 19. A method ofmanufacturing a magnetic recording medium, wherein the magneticrecording medium comprises a magnetic layer comprising ferromagneticpowder and binder on a nonmagnetic support; wherein the magnetic layercomprises multiple nonmagnetic particles having a ratio of a major axislength to a minor axis length, major axis length/minor axis length, ofless than or equal to 1.5; the multiple nonmagnetic particles arepresent in the magnetic layer in a state where, when a depth to whicheach of the multiple nonmagnetic particles is embedded in the magneticlayer in observation of a sectional image picked up by a scanningelectron microscope is denoted as b and a thickness of the magneticlayer is denoted as t, an average value of a ratio of b/t is less thanor equal to 0.9; and a number of protrusions greater than or equal to 5nm in height is greater than or equal to 800 and a number of protrusionsgreater than or equal to 20 nm in height is less than or equal to 20 asmeasured by an atomic force microscope per an area 40 μm×40 μm on amagnetic layer side surface of the magnetic recording medium; and themethod comprises formation of a magnetic layer, wherein the formation ofa magnetic layer includes: coating and drying a magnetic coatingfilm-forming composition comprising ferromagnetic powder and binder, butnot comprising nonmagnetic particles with a ratio of a major axis lengthto a minor axis length, major axis length/minor axis length, of lessthan or equal to 1.5, on a nonmagnetic support, either directly or overone or more other layers to form a magnetic coating film; coating anddrying an overcoat liquid, comprising multiple nonmagnetic particleshaving a ratio of a major axis length to a minor axis length, major axislength/minor axis length, of less than or equal to 1.5 and solvent, onthe magnetic coating film to dispose the nonmagnetic particles on themagnetic coating film; and pressing into the magnetic coating film thenonmagnetic particles that have been disposed to form a magnetic layerthat comprises ferromagnetic powder and binder and in which thenonmagnetic particles are present in the state.